The invention relates generally to electromagnetic clutches, and more specifically to an electromagnetic clutch assembly which finds particular application and utility in motor vehicle, operator selectable two/four wheel drive systems.
A common option on modern cars, light trucks and off road vehicles is operator selectable (part-time) four wheel drive. In addition to conventional two wheel drive train components, these systems include a transfer case for directing a portion of the drive train power to the front wheels, a front prop shaft, a front differential, a pair of live front axles and one of a number of clutch configurations and dispositions for selectively coupling drive energy to the front wheels.
Both the emphasis upon lighter weight, more fuel efficient vehicles and the popularity of such part-time four wheel drive systems has prompted the development of numerous designs for components of such systems, particularly the clutch mechanisms.
One style of clutch commonly used to selectively drive the front wheels of the motor vehicle is a disc pack type clutch. Such a clutch is generally illustrated in U.S. Pat. No. 3,507,374. While such devices exhibit good reliability and power handling ability, they are generally somewhat bulky, resulting in a transfer case housing that is unwieldy or simply unadaptable to smaller vehicles. Secondly, a hallmark of such clutch configurations is their rapid, positive engagement. In part-time four wheel drive power trains, such engagement, when accomplished on the fly, that is, while the vehicle is in motion, may create a transient shock in the power train and a corresponding accelerative or decelerative jerk of the vehicle readily sensed by the driver and occupants. Clearly, then, one desirable feature of such clutches, especially those intended for on the fly engagement, is smooth engagement.
Another desirable feature of such systems are designs which minimize the number of components and thus the weight of the overall four wheel drive system. U.S. Pat. No. 4,667,767 teaches a complex four wheel drive system, having a conventional drive shaft to the rear wheels and further includes a transfer case adjacent the rear differential and prop shafts which extend from the rear mounted transfer case forwardly to the front axle and differential. A centrally disposed transfer case would eliminate several components of this design. A centrally disposed transfer case must, however, be relatively compact and this, in turn, necessitates a compact clutch design.
It is apparent from the foregoing discussion, as well as an assessment of the current state of four wheel drive systems, that improvements thereto are both desirable and possible.